The present invention is a method for controlling cure initiation time and curing time of a platinum group metal curable fluorosilicone composition. The method comprises in a platinum group metal curable fluorosilicone composition comprising a cross-linker mixture comprising an alkylhydrogensiloxane and a dialkylhydrogen perfluoroalkylethylsiloxane controlling the weight ratio of the alkylhydrogensiloxane to dialkylhydrogen perfluoroalkylethylsiloxane in the mixture within a range of about 0.1:1 to 9:1 to control the cure initiation and curing time of the composition.
It is known in the art that fluorosilicone elastomers maintain desirable physical properties over a wide temperature range and have good resistance to hydrocarbons such as gasoline and engine oil. Therefore, fluorosilicone elastomers are useful as gasket and seals in a wide variety of applications. The ability to mold such gaskets, seals, and other articles from fluorosilicone compositions accurately and quickly is necessary for cost competitiveness of such moldings. For moldings prepared from fluorosilicone compositions to be cost competitive it is necessary that the compositions have a cure initiation time sufficient to allow complete filling of a mold cavity and then for the composition to cure rapidly allowing for a rapid cycle time for the molding operation.
It is also known that fluorosilicone compositions can be cured by an addition-type reaction using a platinum catalyst and an alkylhydrogensiloxane as a cross-linker. Generally, the use of an alkylhydrogensiloxane as cross-linker in fluorosilicone compositions provides for a composition that has a relatively long cure initiation and cure time. Therefore, although such compositions can provide for a complete filling of a mold cavity prior to cure initiation, the molding cycle time is slow.
Ikeno et al., U.S. Pat. No. 4,818,805, discuss the issue relating to the relative slow cure initiation and curing time of fluorosilicone compositions using an alkylhydrogensiloxane as cross-linker. Ikeno et al. describe the use of an unique siloxane cross-linker containing silicon-bonded hydrogen atoms, fluorine substituted organic substituents bonded to silicon, and a divalent organic group with both valencies bonded to separate silicon atoms for curing of fluorosilicone compositions. The siloxane cross-linker described by Ikeno et al. is report to provide for a quick cure initiation and fast curing of fluorosilicone compositions.
Maxson, U.S. Pat. No. 4,732,931 and U.S. Pat. No. 4,857,564 describes platinum curing fluorosilicone compositions comprising a dialkylhydrogen trifluoropropylsiloxane cross-linker having repeating units described by formula ##STR1## where Prf represents a 3,3,3-trifluoropropyl group. The compositions described by Maxson are known to have fast cure initiation and curing times. The disadvantage of these fast cure initiation times is that large or complex molds may not have time to fill completely before curing is initiated. This results in a large number of device rejects.
The present invention provides a method where the cure initiation and cure time of a platinum curing fluorosilicone composition can be tailored over a significant range to allow for accurate and rapid molding of fluorosilicone devices. The method comprises in a cross-linker mixture controlling the weight ratio of an alkylhydrogensiloxane to dialkylhydrogen perfluoroalkylethylsiloxane within a range of about 0.1:1 to 9:1. An important aspect of this invention is the discovery that the ratio of the cross-linkers can be varied within the described range while maintaining acceptable physical properties of the cured fluorosilicone elastomer.